Digital telecommunications across great distances has become a commonly used and widely adopted method of sharing information and ideas. The widespread adoption of facsimile technology, voice and video teleconferencing, and electronic mail has resulted in the formation of new paradigms of business organization. With the modern telecommunications technology, proximity of location is not nearly as relevant as connectivity bandwidth. Working professionals, governments, businesses, and other segments of the economy are reorganizing themselves around the widespread adoption of point to point telecommunications devices.
Essentially, digital telecommunications have proven to be a far more powerful and compelling method of communication than previous, conventional methods. Communication and exchanges between one user and another distant user occur virtually in real time. For example, a fax machine can communicate information and pictures nearly instantaneously, as opposed to conventional mail. Electronic mail allows one user to send numerous document files and large image files to the distant user immediately. The rise of this new paradigm has led to a huge increase in the demand for point to point digital telecommunications.
The problem, however, is that the public switched telephone network through which the majority of this information passes was not designed to handle high bandwidth digital data. The public switched telephone network (PSTN) was primarily designed for point to point voice communication. However, designers of data modems have attempted to make the best use of this situation, but even so, the vast majority of point to point digital communications across the PSTN proceeds at speeds much slower than the theoretical 64 Kbps limit of the PSTN when using digital switching of PCM channels. Users are demanding faster, reliable 64 Kbps connections.
Prior Art FIG. 1 shows a diagram of a typical communications channel 100 in the PSTN. The communications channel 100 includes a user modem 101 sending transmit data and downloading receive data to and from a user on the left of FIG. 1 (not shown). User modem 101 is coupled to a 2-4 wire converter 102, and in turn, via a local loop, is coupled to a second 2-4 wire converter 103. In communications channel 100, the two wire line connecting 2-4 wire converter 102 and 2-4 wire converter 103 is referred to as the local loop. The 2-4 wire converter 103 is coupled to a PCM (pulse code modulation) codec 104. The 2-4 wire converter 103 and PCM codec 104 are often integrated into one piece of equipment referred to as a subscriber line interface card. PCM codec 104 is coupled to a digital switched network 105 portion of the PSTN, and in turn, is coupled to a PCM codec 106. PCM codec 106 is coupled to a 2-4 wire converter 107, then via another local loop to a 2-4 wire converter 108, and subsequently to a remote user modem 109. Remote user modem 109 couples received data and transmit data to and from a user on the right of FIG. 1 (not shown).
The signals from the user on the left of FIG. 1 to PCM codec 104 are analog signals. The signals from the user on the right of FIG. 1 to PCM codec 106 are also analog signals. However, the signals that are transmitted and switched across the digital switched network 105 are digital. The analog signals are converted into corresponding digital signals by the digital to analog converters (DACs) and the analog to digital converters (ADCs) of PCM codec 104 and PCM codec 106. In the case of voice communications, the analog signal is a voice waveform and its corresponding digital signal in the PSTN is a sampled representation of the voice waveform, after it is subjected to A (or .mu.) law compression. In the case of conventional data communication, the analog signal is an analog waveform onto which is modulated the digital data. The corresponding digital signal within the PSTN is a sampled representation of this analog waveform (also after A or .mu. law compression).
The 2-4 wire converter 103 and the PCM codec 104 comprise the major portions of a subscriber line interface card. Similarly, 2-4 wire converter 107 and the PCM codec 106 also comprise a subscriber line interface card. The subscriber line interface card (SLIC) functions by converting the user's analog signal into a digital representation of the sampled analog signal after A or .mu. law compression. Once converted, the digital signal is transmitted and switched through the digital switched network 105 of the PSTN. At the destination, the digital signal is coupled to another SLIC where it is converted from digital form back into analog form.
If the analog signal is a voice signal, the analog signal is used to drive the speaker of a telephone, recreating the user's voice. If the analog signal is a data signal (e.g., a V.21 modem signal), the digital data is extracted from the analog signal for use by the receiving user's computer system by user modems 101 and 109. Communications channel 100 functions adequately when used to transmit and receive analog information (e.g., voices). However, communications channel 100 proves inefficient when utilized to transmit high bit rate data streams. The present state of the art allows a maximum data rate of 40-50 Kbps for K56-type modems (server to client direction only). The limitation for the latter being caused mainly by the A or .mu. law expansion process at the local exchange.
One of the major bottlenecks to high speed data transmission through a communications channel is the presence of prior art SLICs. For example, one of the primary challenges in designing 56 Kbps PCM (pulse code modulation) type modems is accounting for the detrimental effects of the DACs and ADCs contained within the SLICs.
Prior Art FIG. 2 shows a typical prior art SLIC 200. SLIC 200 includes 2-4 wire converter 103 and PCM codec 104. PCM codec 104 includes an 8 bit A law ADC 201 and an 8 bit A law DAC 202. The 8 bit A law ADC 201 transmits information upstream to the digital switched network as a 64 Kbps digital data stream. The 8 bit A law DAC 202 receives downstream information as a 64 Kbps data stream. While SLIC 200 includes A law DAC 202 and A law ADC 201, those skilled in the art understand that alternatively, SLIC 200 could include a .mu. law DAC and .mu. law ADC instead.
As is well known in the art, in the downstream direction (e.g., from an internet service provider to a user), 8 bit A law DAC 202 restricts the number of signaling levels from a theoretical maximum of 256 voltage levels. The number of levels actually available are much lower. Many of the signaling levels around the origin are spaced too closely together and are unusable due to DC offset voltage levels and other signal characteristics of the 8 bit A law ADC 202. This forces the user modem 101 to use larger amplitude levels with a corresponding increase in the transmit signal power, as the bit rates increase. Thus, one of the factors limiting the maximum bit rate is the maximum allowable power levels on the local exchange lines. Those desiring more detailed information regarding A law and .mu. law codec standards are directed to "ITU-T G.711, PULSE CODE MODULATION (PCM) OF VOICE FREQUENCIES, International Telecommunications Union" which is incorporated herein as background material.
One solution presently available to the user is to convert to ISDN (integrated services digital network). However, this requires an expensive hardware change to the SLIC 200 (e.g., the removal and replacement with a new SLIC) and software and other hardware changes to PSTN such that the user's local loop is no longer viewed by the PSTN as a regular analog line. Consequently, ISDN is presently an expensive option for the user. The expense is primarily due to the software changes required within the PSTN and also the fact that an ISDN connection requires a total of 144 Kbps (e.g., basic ISDN 2B+D connection) in both directions, compared to only 64 Kbps full duplex for an analog line. Consequently, ISDN imposes more demands on the local exchange's limited available digital throughput to the next level of switching in the PSTN (e.g., digital switched network 105). The higher 144 KBPS data rate also requires expensive engineering time to check the suitability of the local loop for supporting this rate and to carry out possible modifications to the loop, such as the removal of loading coils.
Thus, what is required is a solution which improves the data transfer bandwidth for users coupled to the PSTN. The required system should require a straight forward and economical change to the hardware of the PSTN and should be relatively easy to implement in comparison to ISDN. The required system should increase the efficiency of modems coupled to the PSTN. Additionally, the required system should be compatible with ordinary analog voice communication and the PSTN should view the upgraded user as an entirely conventional voice-grade service. The present invention provides a novel solution to the above requirements.